As for a generally used water level measurement method, there are two methods, i.e., a method using a water gauge and a method using image processing. The method using a water gauge can measure a water level with high accuracy by transmitting a pressure measured by a detector installed in water to a measuring board via a repeater. The method using image processing which is considered as a method replacing the method using a water gauge proposes various techniques (see, e.g., Patent Document 1). FIG. 2 shows an outline of water level measurement using image processing. A monitoring camera 500 set on a river RV side images a predetermined area CA of the river RV which includes a structure OB. In the measurement method using image processing, a water level is measured by detecting a boundary WL with respect to a water surface W from an image B0 of the monitoring camera 500 through image processing.
In a disaster prevention system, when it is determined that a water level of a river which is constantly measured by the above methods reaches a dangerous level, for example, alarm is notified to a monitoring center, and the monitoring center that received the notification gives an evacuation instruction to neighboring residents.
Patent Document 1: Japanese Patent Application Publication No. 2008-057994
However, the above-described two water level measurement methods have drawbacks. First, the method using a water gauge which has been introduced for river monitoring has two major drawbacks. The first drawback is that it is not difficult to observe fine water level variation between an upstream side and a downstream side of a river since a small number of water gauges is installed. When a monitoring camera and a water gauge are distanced from each other, an error occurs in the water level appearing on the camera and a measurement value of the water gauge. Although this drawback can be solved by increasing the number of water gauges, it is expected that a high installation cost and a high maintenance cost are required, which is not realistic. Second, when an observer checks only a measurement value of the water gauge, it is difficult to display a state of a river corresponding to a current measurement value. Accordingly, it is difficult for the observer to make accurate judgment, and an evacuation instruction may be delayed.
The first drawback can be solved by using an existing monitoring camera. This is because monitoring cameras are often installed at a short distance in major rivers managed by the Ministry of Land, Infrastructure and Transport of Japan. If the water level can be measured from this camera image, it is possible to observe fine water level variation of the river and minimize an equipment installation cost. The second drawback can be solved by displaying a current water level on a camera image. By drawing the water level line on the image, it is easier to visually judge dangerous situation, and a prompt evacuation instruction can be made. Due to these advantages, the demand for water level measurement by image processing using an existing monitoring camera is increasing.
However, the water level measurement using image processing is disadvantageous in initial setting and measurement accuracy. First, in regard to the initial setting, it is necessary that an expert sets an angle of view in order to perform image processing with high accuracy. This is disadvantageous in that the number of working processes at the time of introduction is increased and it is difficult to change the angle of view after the introduction. In regard to the measurement accuracy, the accuracy may deteriorate depending on an environment of a measurement target area. The accuracy deteriorates when a water surface cannot be detected accurately and when motion other than water flow inflicts an adverse effect. The water level measurement using image processing is realized by finding a boundary line between the water surface and a pier or the like above the water surface. Therefore, if the water surface cannot be detected accurately, the accuracy deteriorates. FIG. 2 shows an example of an image B1 having reflection R of the structure OB on the water surface W. If the structure OB such as a bridge pier or the like is reflected on the water surface W, the water surface W may be determined as the structure OB. Therefore, a new technique for measuring a water level through image processing is required.